3 drying methods for water-based coating of furniture

Coating drying of water-based coatings is the need to ensure the quality of coating, is an indispensable process of the whole coating process, directly affecting production costs and efficiency.
conditions under which the coating is dried has a significant impact on the quality of the final product. The coating drying is unreasonable, the paint film will produce poor gloss, orange peel, pinhole and other defects, serious will exist in the paint film internal stress, so that the paint film adhesion is reduced; Cracks in use, loss of protective decorative effect, it is difficult to ensure the stability of paint film performance and access to good decorative quality

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At the same time, coating drying is a frequent and time-consuming process, so in modern production, how to speed up coating drying is not only related to shortening the production cycle and saving production area, but also to achieve construction continuity and automation must solve the technical key issues.
Refore, in order to ensure the quality of coating and improve the efficiency of painting, it is of great significance to study the drying method of water-based coating, an important way to improve the drying efficiency, and an important problem in the development of production, which has important application value.
furniture water-based paint is water-based paint. The solid content of water-based coatings is very low, generally only 20%-30%, containing about 70% of the moisture. Therefore, water-based coatings in the drying process is mainly the volatile moisture. The film-forming process and moisture evaporation process of water-based coatings are described below.
1. The film-forming process of furniture water-based coatings
the film-forming process of water-based coatings is a molecular chain condensation phenomenon, which can generally be divided into three steps: water evaporation, particle deformation and particle merger.
construction, the first is the volatile moisture, when the latex particles of water-based coatings account for 74% of the glue layer (volume), the particles close to each other and reach a dense filling state; Secondly, the water continues to evaporate, polymer particles deformation, capillary pressure is higher than the anti-deformation force of polymer particles, the pressure between particles, the more the medium volatility, the greater the pressure, and then condensation, fusion to form a continuous coating film; Finally, the moisture continues to evaporate, when the pressure reaches the molecular chain in each particle can spread to another particle molecular chain, the polymer molecular chain gradually spread to each other step by step, so that the membrane is uniform, the curing and drying process is completed.
, water evaporation process research
Theoretical analysis, water-based coating moisture evaporation process in line with the table and vertical drying theory. The theory holds that the evaporation of moisture when water-based coatings are filmed can be divided into three stages, the first stage, resin particles carry out irregular Brown movement, moisture and in pure water along the same water-air interface at the same volatilization rate; After entering the second stage, the suspended medium on which the resin particles depend is reduced, so that the resin particles are close to each other, accumulation and clustering, causing the water-air interface to contract, so that the total area of the water-air interface is reduced, and the rate of moisture volatilization decreases rapidly; In the final stage, the rate of moisture volatilization is decreasing, and the remaining moisture spreads through the capillary pipes in the membrane to the coating film - the air surface evaporates. The whole process is gradually developed from the surface of the resin particles to the inside, and the rate of moisture volatilization is reduced by the decreasing water and the transfer of water from the gap between particles to the transmission through polymers.
Thye, drying mode
wood drying process is the process of water gradually excluding the wood interior, and furniture water-based coating drying is to a large extent the water gradually excluded from the coating internal process, in a sense, the two have some similarities. Therefore, in further study of wood furniture with water-based coating drying methods, we can learn from the relatively mature wood drying methods. In the field of wood drying, natural drying, hot air drying, microwave drying, ultraviolet drying, infrared drying, solar drying, high frequency drying, combined drying and other methods are theoretically applicable to the drying of water-based coatings, but it is necessary to study the specific technical parameters of various drying methods in water-based coating drying applications.
main factors affecting the drying speed of water-based wood coating are the ambient temperature, the size of relative humidity, the thickness of the paint film and the speed of air flow. Wu Zhongyue and others believe that water-based wood coating to achieve assembly line coating, can speed up the drying speed, improve the hardness of the coating film, reduce VOC and avoid the "slow drying" brought about by many disadvantages. Cai Jiabin and others have studied the application of microwave drying in polyurethane water-based wood coating, and the results show that after microwave drying, the moisture in the water-based coating film is removed by 90%, while the drying of the oven during the same time can only remove about 50% of the total moisture. Microwave drying immediately after the coating film reached the dry, can be polished, can be stacked, packaging.
different drying methods are obviously different in the drying speed and film quality of water-based coatings, the following is an overview of the various drying methods of water-based coatings.
1
naturally dry
the evaporation potential heat of water is very large, reaching 2457.7KJ/Kg, water-based coating moisture evaporation needs to absorb a lot of heat. The volatility of water is affected by atmospheric temperature and humidity and wind speed, the volatility rate increases with the increase of temperature, decreases with the increase of air humidity, increases with the increase of wind speed and becomes larger, because the temperature increases, the molecular heat movement is more intense, and the liquid water molecules are more likely to get rid of the attraction of other water molecules and become steam molecules. However, with the increase of air humidity, the proportion of water vapor in the air increases, the sub-pressure of water vapor increases, so that more water vapor molecules become liquid water molecules in a unit of time, so the evaporating speed of water decreases with the increase of air humidity. The higher the wind speed, the more water molecules are removed from the wave surface in unit time, so the faster the volatility.
natural drying has the advantages of simple method and wide application, but at the same time, it has the disadvantage of slow drying speed. Under natural conditions, temperature and humidity and wind speed are constantly changing, and the drying speed and film quality are unstable. If the high temperature, high humidity or humidity is relatively large, the coating is prone to whitening and the drying speed is slow. At low temperatures, the drying speed is very slow; Especially at 5 degrees C, water-based coatings are difficult to film. These are the important reasons that restrict the popularization and application of water-based coatings.
2
health air drying
hot air drying is the use of conflow principle, the temperature of 40 to 60 degrees C of hot air as the carrier of heat, the heat energy to the surface of the workware coating, the coating absorbs energy and curing into a membrane of heating and drying method. Electricity or steam is commonly used as a heat source to heat the air first, and heat is passed from hot air to the coating surface in the form of a flow, allowing the coating to dry quickly.
is drying with hot air, the hot air around the coating is the heating medium. The coating always has a certain thickness, heat from the coating surface to the boundary of the middle layer, it takes a certain amount of time. The speed at which heat is transmitted depends on the thickness of the coating and its thermal conductivity. Therefore, when conflow heating, the coating surface is always heated first. In the early stages of drying, the surface moisture evaporates most strongly, and the curing of the coating begins at the surface and then gradually extends to the bottom layer, causing the bottom layer to dry at last.
the use of hot air drying process, can obviously speed up the coating drying speed, and has the characteristics of adaptability, is a more widely used drying form. In large-scale furniture factory, hot air drying line is the main form of forced drying of solvent-based coatings, changed to dry water-based coatings, because of the long drying time of water-based coatings, on the one hand, the need to study drying equipment, on the other hand, the need to study the drying process. Temperature, humidity, workstation speed (drying time) and ventilation conditions throughout the drying line are essential to ensure drying quality and final paint film quality.
3
Microwave Drying
microwave refers to the wavelength of 1mm-1m, the frequency of 300MHz-300GHz, penetrating electromagnetic waves, the commonly used microwave frequency is 915-2450MHz.
microwave heating uses the principle of medium loss, and the dielectric constant of water is much larger than that of dry matter, and the vast majority of the energy released by electromagnetic fields is absorbed by the moisture in the coating.
microwave field periodically changes the direction of the added electric field at a high speed of hundreds of millions of times per second, causing the water molecules to swing rapidly, producing significant thermal effects that cause the temperature inside and on the surface of the coating to rise rapidly at the same time. The advantage of microwave heating is that the drying speed is particularly fast, and different substances have selective absorption for microwave. There are no shape requirements for dried objects; The heating of the coating film is very uniform, there is no temperature gradient, the thick film can be dried. For water molecules, microwaves with a frequency of 2450MHz have the best balance of energy conversion and loss. Microwaves at this frequency can penetrate a 30mm thick layer of water and can be used to dry water-based coatings of various thicknesses.
4
UV
for water-based UV wood coatings, can be used UV-cured drying. The so-called UV curing refers to the water-based UV wood coating in the wavelength of 300-400nm of ultraviolet radiation curing drying method.
In water-based UV coatings contains a small amount of photosensitive agents, in ultraviolet radiation, photosensitive agents absorb a specific wavelength of ultraviolet light, decomposition produces an active group, triggering a polymerization reaction of film-forming substances, forming a mesh structure and making the coating cured. Ultraviolet curing has the characteristics of fast coating curing speed and good film quality. But. This method can only be used to dry water-based UV coatings, and can only be dried into flat-panel furniture coating board.
5
infrared drying
infrared curing can be used to dry the coating of water-based wood coatings. The so-called infrared curing actually refers to the coated furniture plate and its coating in the infrared light, absorbing radiant energy and converted into thermal energy, so as to achieve the curing of the coating.
infrared is an invisible ray, between visible light and microwave, with a wavelength of 0.72-1000um. According to the wavelength range, can be divided into "near", "medium" and "far" infrared, commonly used far-infrared drying coating.
infrared curing has the advantages of fast curing speed, rapid heating, good curing quality, but when using infrared heating drying coating, the coating has a clear temperature gradient, its drying is extended from the surface to the inside, which makes it not suitable for drying thick coating film; Infrared drying can only heat the area that infrared can shine on, and cannot be used to dry three-dimensional objects.
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